A duplex scanning device is widely used for scanning images on both sides of a document. A typical duplex scanning device comprises a flatbed image scanning portion and an automatic document feeder arranged above the flatbed image scanning portion. Generally, there are two mechanisms for performing the duplex scanning processes. The first mechanism uses a scanning module in the flatbed image scanning portion for scanning one side of the document while using another scanning module in the automatic document feeder for scanning the other side of the document. The second mechanism uses only one scanning module in the flatbed image scanning portion to scan both sides of the document.
Since the first mechanism has two scanning modules, the scanning speed for performing a duplex scanning operation is relatively larger than the second mechanism. However, the cost of the first mechanism is higher than that of the second mechanism. Consequently, the second mechanism is prevailing. When the second mechanism with the single scanning module performs a duplex scanning operation, the document needs to pass through the scan region for three times. As a consequence, the duplex scanning operation by the second mechanism is also referred as a three-pass scanning operation. When the document is transported across the scan region for the first time, the scanning module scans a front side of the document. When the document is transported across the scan region for the second time, the scanning module scans a back side of the document. Finally, the document is transported across the scan region for the third time to rearrange each document into the same order as that of the original stack.
Referring to FIG. 1, a schematic cross-sectional view of a conventional duplex scanning device 100 operated by the second mechanism with the single scanning module is illustrated. First of all, a document 111 to be scanned is placed in a sheet feeding tray 102. The automatic document feeder 101 transports the document 111 into a first path 104. Next, the document 111 enters a scan region 107 through a second path 106. When the document is transported across the scan region 107, a scanning module 105 scans a front side of the document 111. Then, the document 111 is transported across a third path 108 and a guiding rod 109 to a reverse region 112. The guiding rod 109 is switched between a first position leading the document 111 to the reverse region 112 and a second position leading the document 111 to the discharging tray 103. In the reverse region 112, the document 111 is transported across the guiding rod 109 to a fourth path 110, and then enters the scan region 107 through the second path 106. When the document 111 is transported across the scan region 107, the scanning module 105 scans a back side of the document 111. Then, the document 111 is transported across the third path 108 and the guiding rod 109 to the reverse region 112. Next, the document 111 in the reverse region 112 is transported across the guiding rod 109 to the fourth path 110, the second path 106, the scan region 107 and third path 108. When the document passes through the scan region 107 for the third time, the scanning module 105 does not scan the document 111. Finally, the guiding rod 109 is switched to the second position, thereby leading the document 111 to the discharging tray 103.
As previously described, the duplex scanning device with the single scanning module includes one flatbed image scanning portion and one automatic document feeder. The scanning module in the flatbed image scanning portion is responsible for performing the duplex scanning operation. In other words, for performing the duplex scanning operation, the automatic document feeder fails to be operated alone but should be cooperated with the flatbed image scanning portion.
Please refer to FIG. 1 again. During the process of performing the duplex scanning operation, the scanning module 105 should be moved under the scanning window (i.e. the scan region) 107 such that the light emitted from the scanning module 105 can be projected onto the document 111 passing over the scanning window 107. Therefore, it is required to reserve an additional operative space 120 for accommodating the scanning module 105. Due to the operative space 120, the length and the overall volume of the duplex scanning device 100 are increased.
In views of the above-described disadvantages resulted from the prior art, the applicant keeps on carving unflaggingly to develop an improved automatic document feeder for duplex scanning according to the present invention through wholehearted experience and research.